1. Field of Invention
This invention relates to an electrophotographic printing machine, and more particularly to a drum cap grounding mechanism for a drum cap charging mechanism in the drum cap, which drum cap forms a photosensitive drum mechanism together with a photosensitive drum in the electrophotographic printing machine.
2. Description of Related Art
The photosensitive drum mechanism of a conventional electrophotographic printing machine is as shown in FIG. 2 (PRIOR ART).
In FIG. 2 (PRIOR ART), various parts of the figure are designated by reference numerals 1-5: a web-shaped photosensitive material 1 (a belt-shaped thin film about 90 .mu.m in thickness); a photosensitive drum 2 on which the photosensitive material is wound; a take-up roll 3, arranged inside the photosensitive drum, for winding the photosensitive material; a supply roll 4 also arranged inside the photosensitive drum, for supplying the photosensitive material when necessary; and a metal drum cap 5 provided for the photosensitive drum, to seal the interior of the photosensitive drum.
FIG. 3 (PRIOR ART) is an explanatory diagram, drawn partly as a sectional diagram, for a description of the function of the conventional photosensitive drum mechanism. In FIG. 3 (PRIOR ART), various parts are designated by reference numerals 1-10: reference numerals 1-5 designating the same parts designated in FIG. 2 (PRIOR ART), plus, a drum cap charging mechanism 6, a photosensitive material charging mechanism 7; a developing mechanism 8; and an exposing mechanism 10.
A conventional printing operation for the photosensitive material 1 will be described with reference to FIG. 3 (PRIOR ART).
First, the photosensitive material 1 is supplied from the supply roll 4 and led out of the drum 2 through a first gap between the drum and the drum cap 5. The photosensitive material thus protruded is wound counterclockwise on the drum about one turn. The front end portion of the photosensitive material thus wound is inserted into the drum through a second gap between the drum and the drum cap, and wound on the takeup roll 3 until the photosensitive material is stretched tight. Thus, the photosensitive material is fixedly wound on the drum.
In this condition, the photosensitive drum is turned counterclockwise. Therefore, the photosensitive material wound on the drum is first charged by the photosensitive material charging mechanism 7, and then exposed by the exposing mechanism 10, and finally developed by the developing mechanism 8. Thus, the printing operation for the photosensitive material is accomplished.
The image developed by the developing mechanism is erased every revolution of the drum, and a new printing image is formed. However, when the number of revolutions of the drum reaches about 10,000, the photosensitive material will have deteriorated in photosensitivity, and therefore it is necessary to replace it with new photosensitive material. The replacement is carried out by using the supply roll and the takeup roll in the above-described manner.
The drum cap is metal, as described above, and its function is to prevent the entrance of foreign materials such as toner, developing agent and paper powder into the drum during rotation.
During the rotation of the photosensitive drum mechanism, the drum cap is charged to a high potential by the photosensitive material charging mechanism. If the potential of the drum cap is decreased, then foreign materials such as toner are liable to stick to the drum cap. In order to overcome this difficulty, the drum cap is provided with the above-mentioned drum cap charging mechanism so that, when the drum cap, thus charged, passes through the developing mechanism, the potential of the drum cap is maintained equal to the photosensitive material surface potential.
As described above, when the photosensitive drum mechanism is in operation, the drum cap is held at a high potential by the drum cap charging mechanism provided for it.
Conventionally, the drum cap at high potential is placed in natural discharge when the photosensitive drum mechanism stops rotating. Therefore, after the printing operation is suspended, the drum cap stays at a high potential for a time. This results in a problem when maintenance work needs to be performed after the printing operation is suspended, because the operator may receive an electric shock when touching the drum cap.